Magnetic flowmeters for low flow rates



June 11, 1968 E. D. MANNHERZ ETAL 3,387,492

MAGNETIC FLOWMETERS FOR LOW FLOW RATES Filed May 2. 1966 iO H16INVENTORS FL/76.? D MAN/mm2 Cmes P @of/Mmm BY jar/ley #ff/; //Mmm UnitedStates Patent O 3,387,492 MAGNETIC FLOWMETERS FOR LOW FLOW RATES ElmerD. Mannherz, Southampton, Charles P. Rohmann, Hatboro, and HenryHermanns, Ardsley, Pa., assignors to Fischer e Porter Co., Warminster,Pa., a corporea tion of Pennsylvania Filed May 2, 1966, Ser. No. 546,785Claims. (Cl. 73-194) ABSTRACT OF THE DISCLOSURE A magnetic iiowmeteradapted to accurately measure low flow rates, the meter including afiller rod of insulating material which is inserted in a pipe sectionhaving a pair of electrodes disposed at diametrically opposed points,the rod having two parallel channels formed longitudinally therein atdiametrically opposed positions er1- compassing the electrodes, wherebythe fluid to be measured iiows through these channels which serve toreduce the effective cross-sectional area of the pipe section thereby toincrease the velocity of fluid therein.

This invention relates generally to electromagnetic flowmeters, and moreparticularly to improved fiowmeter structures adapted to provideaccurate measurements at low liow rates.

The magnetic iiowmeter is adapted to measure volume rates of thosefluids which present difficult handling problems, such as corrosiveacids, sewage, slurries, detergents, and the like. In a magneticiowmeter, a uniformly distributed magnetic field is generated which ismutually perpendicular to the longitudinal axis of the meter pipethrough which the fluid flows and to the axes of the meter electrodes.Since the velocity of the iiuid is directed along the longitudinal axisof the pipe, the voltage induced within the fluid will be perpendicularto Iboth the velocity of this iiuid and the flux linkages of themagnetic field. Thus the metered fluid constitutes a series of fluidconductors moving through the magnetic field. The more rapid is the rateof iiow, the greater the instantaneous value of the voltage establishedat the electrodes.

Conventional owmeters make use of a flanged stainless steel pipesection, the inner pipe and iiange surfaces being lined with aninsulating material such as Teflon or vitreous enamel. Two cylindricalelectrodes are mounted at diametrically opposed positions in the centralportion of the pipe section, and are completely insulated from the pipeby bushings. The end surfaces of the electrodes are flush with the innersurface of the insulating liner and come in contact with the fluid to bemetered. A field coil assembly is provided consisting of twosaddleshaped magnet coils surrounded by a box-sh aped laminated ironcore, which in turn completely encompasses the central portion of theflowmeter. The iield coil assembly is designed to produce a linear anduniform magnetic field through the metering section.

The instantaneous voltage developed across the meter electrodesrepresents the average iiuid velocity of the flow 4profile passing theelectrodes at a given moment, and the iiowmeter will produce an outputsignal equal to a continuous average tiow rate regardless of whether theiiow profile is laminar or turbulent. A liowmeter output signal isstably and linearly proportional to the volumetric rate of flow throughit. However, to obtain such accurate results, the metering pipe must becompletely filled at all times. The lflowmeter will continue to operatewith full flow output signals as long as the metering fluid level issufficient to maintain contact with the meter electrodes. Since thevolume rate is proportionally related to liuid velocity by means of theconstant area or diameter of the meter pipe section, a partially filledpipe incurs error in a direct relation of the fluid area to the pipearea.

3,337,492 Patented .lune 1l, 196'I With low flow rates, it is thepractice to use flow pipe sections of minimum size to insure filling thepipe. There is, however, a practical physical limitation to the minimumsize iiowmeter which can ybe built. This limitation is a function of thematerial of the liner as well as the method used for sealing theelectrodes. In practice, it has been found that 5/32 diameter is thesmallest-size, Teiion-lined meter which can be constructed, while 1/10"diameter is the smallest vitreous enamel lined meter it is feasible tomake. For very low rates, even these sizes are too large for accuratemeasurements.

Accordingly, it is the main object of this invention to provide amagnetic flowmeter capable of accurately measuring low iiow rates.

More specifically, it is an object of the invention to provide aflowmeter using a pipe section of minimum size and having a sensitivityfar greater than priorart structures of the same dimensions, whereby itis possible to measure flow rates which are but a fraction of the rateswhich heretofore could accurately be measured. Thus the invention hasmade it possible to increase the sensitivity of a 5/32 meter by a factorof 4 and thereby measure ow rates one-fourth as small as the minimumlisted for this size.

Brieiiy stated, these objects are attained vby inserting into the pipesection of the iiowmeter an insulating rod which reduces the effectivecrosssectional area of the section and thereby increases the flowvelocity therethrough, without in any way interfering with theelectromagnetic field established across the pipe section.

For a better undertsanding of the invention as well as other objects andfurther features thereof, reference is made to the following detaileddescription thereof to be read in conjunction with the accompanyingdrawing, wherein:

FIG. l is a cross-section taken through a magnetic flowmeter pipestructure in accordance with one embodiment of the invention;

FIG. 2 is a longitudinal section taken in the plane indicated by line2--2 in FIG. l;

FIG. 3 is a schematic diagram of the flowmeter structure shown in FIGS.l and 2;

FIG. 4 yis a perspective view of a magnetic flowmeter pipe section inaccordance with another embodiment of the invention; and

FIG. 5 is a section taken through the plane indicated by line 5-5 inFIG. 4.

Referring now to the drawings, and more particularly to FIGS. l and 2,there is shown a magnetic iiowmeter in accordance with the invention foraccurately measuring low iiow rates, the meter including a metal pipesection 10 of circular cross-section, made of stainless steel or othersuitable material. Inserted in pipe section lil is a tubular insulatingliner 11, which may be of Teflon or other material which is non-reactivewith the iiuids being measured. Two cylindrical electrodes 12 and 13 aremounted at diametrically opposed points in the central portion of thepipe section, the end surfaces of the electrodes having a curvatureconforming to that of the liner. The signal established between theelectrodes is picked up by suitable leads 1d and l5.

Received within liner 11 is a filler rod 16 of plastic material, thediameter of the rod being substantially equal to the internal diameterof the liner 11i whereby the rod telescopes snugly therein. Cutlongitudinally into the rod at diametrically opposed positions along thefull length thereof, are two identical grooves 17 and 18, preferablyhaving a rectangular cross-section. Grooves 17 and 18 deline a pair ofnarrow iiow channels which encompass electrodes 12 and 13 and lie inparallel relation to the longitudinal axis of the pipe section.

The dimensions of grooves 1,7 and 18 are such as to reduce the effectiveiiow area to approximately one-tenth of the flow area defined by liner11. The invention is by no means limited to this specific reduction ineffective fiow area, and in practice, a greater or smaller reduction maybe made, depending on the parameters of the system and the anticipatedfiow rates to be measured. It is to be understood that should the meterbe constructed with a nonmetallic pipe section, such as one made offiberglass, then a liner is unnecessary, and the filler rod for reducingthe effective iiow area can be inserted directly into the pipe section.

Transversely positioned within plastic rod 16 is a nontmagneticelectrically-conducting plug 19 whose longitudinal axis is aligned withthe axes of the electrodes. Plug 19 acts electrically to interconnectthe flow channels and thereby complete the electrical path between theelectrodes. The electrodes may be made of stainless steel, or for highlycorrosive uses, platinum electrodes may `be employed. The choice of plugmaterial similarly depends on the chemical nature of the fluids beingmeasured.

As shown in FIG. 3, a uniform -magnetic field is produced in a planeperpendicular to the flow path of fluid through channels 17 and 13, bymeans of a pair of electromagnetic coils and 21. The electromagneticarrangement as such is well known and is disclosed in greater detail inU.S. Patent to Head, 3,005,342. The excitation of the coils ispreferably with a commercially available sixty-cycle alternatingvoltage, although other frequencies may be used.

Voltage generated by the flow of fiuid in channels 17 and 18 passingthrough the electromagnetic field is detected by electrodes 12 and 13whose positions are normal to the direction of the field. Channels 17and i8 are insulated electrically from each other, but plug 19 providesan electrical path therebetween. The operation of the meter is based onFaradays law of electromagnetic induction, and as the fluid movestransversely through the uniform magnetic field, a voltage is inducedtherein which is proportional to the rate of fluid fiow, as long as thechannels are completely filled at all times.

Because of the pastic rod in the pipe section acts to increase thevelocity of fluid through the metering section, the sensitivity of themeter and the resultant signal is enhanced. As pointed out previously,in practice this sensitivity may be increased -by a factor of 4 andhigher. Alternatively, it is also possible to attain an increase insensitivity by supporting an insulating rod concentrically Within thepipe section whereby the fluid flows through an annular channel, inwhich event it is not necessary to provide a plug :to complete theconnection between two electrically isolated channels. But in thisarrangement, ribs or other means called for to support the insulatingrod, form an obstruction in the annular flow channel.

Another technique which can be used to raise the sensitivity of afiowmeter so that low flow rates may be accurately measured, is bychanging the shape of the fiow area in the metering section. Inconventional metering pipes of circular cross-section, the equation forthe signal generated by fiuid fiow is:

S=Bdv (1) where S=signal in volts B=flux density, Webers/in.2 d=distancebetween electrodes in inches v=fluid yveloicty in inches per second Theflow equation is:

i But for a pipe whose flow area has a rectangular crosssection, whosewidth, measured along the electrode axes, is 111, and whose height is[1, the equation for flux signal becomes:

Hence by constructing a meter in which the height h of the fiow tube isconsiderably smaller than d, the distance between electrodes, aconsiderable increase in sensitivity `may be attained.

Thus, as shown in FIG. 4, a pipe section 22 of circular cross-section isprovided, which section may be lined. The central portion 23 of the pipesection is flattened so that its height h is much smaller than its widthw. Hence the signal developed between the electrodes 24 and 25 placed atthe opposing sides in the flattened portion 23 is considerably strongerthan that attainable with a pipe of circular cross-section. Theelectromagnetic coils are mounted on the flattened portion in the usualmanner.

While there have been shown and described preferred embodiments ofmagnetic liowmeters in accordance with the invention, it will beappreciated that many changes and modifications may be made thereinwithout, however, departing from the essential spirit of the inventionas defined in the annexed claims.

What we claim is:

l. A magnetic fiowmeter for accurately measuring low flow rates, saidflowmeter comprising:

(a) a pipe section,

(b) a pair of electrodes disposed at dametrically opposed points in saidsection,

(c) means to establish a uniform electromagnetic field in a directionnormal to the flow of fluid through said section to induce a voltage insaid fluid which depends on the 'velocity thereof and is detected bysaid electrodes, and

(d) means to reduce the effective cross-sectional area of said sectionto increase the velocity of fluid therein, said means being constitutedby a filler rod of insulating material inserted in said section, saidrod having two parallel channels formed llongitudinally therein atdiametrically opposed positions which encompass said electrodes, wherebythe fiuid tiows through said channels, and

(e) means to form an electrical path between said channels to completethe electrode circuit.

2. A fiowmeter, as set forth in claim 1, wherein said pipe section andsaid filler rod have a circular cross-section and said channels have arectangular cross-section.

3. A flowmeter, as set forth in claim 1, wherein said pipe section is ofmetal and is lined with an insulating tube.

4. A fiowmeter, as set forth in claim 1, wherein said means forming anelectrical path between said channels is constituted by a metallic plugtransversely disposed in said rod in alignment with the axes of saidelectrodes.

5. A flowmeter, as set forth in claim 1, wherein the cross-sectionalarea of said section is reduced by said rod to about O of its originalvalue.

